This invention relates generally to ultrasound imaging systems. More particularly, it relates to a method and apparatus for using multiple transmissions to blend a fundamental signal into a harmonic image to improve the penetration of harmonic imaging without sacrificing image uniformity.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements that transmit an ultrasound beam and then receive a reflected beam from the object being studied. This operation comprises a series of measurements in which a focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Transmission and reception are typically focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam, also known as a scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in a line and driven by separate voltages under separate time delay. By controlling the time delay (or phase) and amplitude of the voltages applied to the individual transducer elements, a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam can be formed. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when an ultrasonic transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each receiving transducer element.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more of the receivexe2x80x94beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is the result of the directivity of the associated transmit and receive beam pair.
The output signals of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
In the prior art, signals used to form an image reside in either a first frequency band or in a second frequency band. The first frequency band comprises the fundamental band and the second frequency band comprises a harmonic of the fundamental frequency band. The second frequency band substantially excludes the first frequency band. A composite image is formed by signals from the second frequency band in the near field and by signals from the first frequency band in the far field. While such method can improve the penetration of the harmonic imaging, it achieves this at the expense of the image uniformity. The image speckle size associated with the first frequency band is usually much bigger than that associated with the second frequency band. As a result, the composite image has a smaller speckle size in the near field and larger speckle size in the far field, which leads to the degradation in the image uniformity.
The current invention is a method and an apparatus for improving the penetration of the harmonic imaging while preserving the image uniformity. It achieves this by using signals from a similar frequency band to form a composite image. More specifically, a near field image uses primarily tissue generated harmonic signal associated with the first transmitting event that has a center frequency of f1. Such tissue generated harmonic signal has a frequency band centered on 2f1. In the far field, fundamental echo signals from the second transmitting event that has a center frequency of f2 are primarily used. Since the center frequency f2 in the second transmitting event is close to 2f1, and there is a significant overlap in frequency band between signals extracted from the first transmitting event and signals extracted from the second transmitting event, a composite image formed from these signals has similar speckle size across the whole image. Because the extracted signal from the second transmitting event is a fundamental component of the received echo, such signal has much larger amplitude than the tissue generated harmonic signal from the first transmitting event. Therefore, adding the fundamental signal from the second transmitting event in the far field improves the penetration of the harmonic imaging without sacrificing the image uniformity.
In short, the method and apparatus of the present invention blends a fundamental signal into a harmonic image to improve penetration of the harmonic imaging. The blending is done in such a way that penetration of a harmonic image is improved without sacrificing the image uniformity. The foregoing and other features of the method and apparatus of the present invention will be apparent from the detailed description that follows.